1. Field of the Invention
This invention relates to a High-Frequency (HF) plasma processing apparatus, which processes a substrate, utilizing plasma generated by HF wave energy.
2. Description of the Related Art
Various surface processes onto substrates are required in manufacturing such electronic devices as LSIs (Large-Scale Integrated Circuits) and LCDs (Liquid Crystal Displays). Apparatuses utilizing plasma generated by an HF discharge are used for such the processes. This kind of apparatuses is hereinafter called “HF plasma processing apparatuses”. Though “HF” usually means frequencies ranging from 3 MHz to 30 MHz, “HF” in this specification broadly means frequencies higher than MF (Medium-Frequency, 300 kHz to 3000 kHz), including VHF (Very-High-Frequency, 30 MHz to 300 MHz) and UHF (Ultra-High-Frequency, 300 MHz to 3000 MHz).
In an HF plasma processing apparatus, a substrate to be processed is held on a substrate holder in a process chamber. An HF discharge is ignited at the space facing the substrate, thereby processing the substrate. In etching the substrate, for example, plasma is generated introducing a fluorohydrocarbon gas into the process chamber. Utilizing reactions with radicals or activated species of fluorocarbon or fluorine, the surface of the substrate is etched. In depositing a film by HF sputtering, plasma is generated by a HF discharge, thereby sputtering a target. There has been well known the chemical vapor deposition (CVD) utilizing a gas-phase reaction in plasma generated by an HF discharge as well.
The described HF plasma processing apparatuses have ever employed frequencies ranging from 3 MHz to 30 MHz, such as 13.56 MHz allocated for industrial use. For enhancing process quality and process speed, however, higher frequencies have the advantage that plasma generation efficiency is improved. Because the probability of electrons to collide with neutral gas molecules is increased, higher frequencies can make the ionization efficiency higher, thus enhancing the plasma-generation efficiency. Enhancement of the plasma-generation efficiency means capability of reducing impurity inclusion by carrying out processes under lowered pressures, or capability of increasing process speed enabled by carrying out processes utilizing higher-density plasmas.
Form the research by the inventor, however, a higher frequency such as 60 MHz might bring the problem that efficiency of power supply to a process gas tends to decrease, resulting in that the power efficiency of a whole apparatus, i.e. process efficiency per input power, is not increased so much.